Needlefree injectors are used as an alternative to needle-type hypodermic injectors for injecting liquid drugs through the epidermis and into the underlying tissues. The usual form of construction for such a device is a syringe having a small discharge orifice which is placed in contact with the skin, and through which the drug is injected at a sufficiently high speed to penetrate the skin of the patient. The energy required to pressurise the drug may be derived from a compressed coil spring, compressed gas, explosive charge or some other form of stored energy.
There are a number of different ways in which the energy may be converted from the stored form into pressure in the liquid. These may include rupturing a seal, so allowing gas to escape from a canister and causing pressure to build up behind a piston which pressurises the gas. Alternatively, a gas may cause a ram to accelerate across a gap, prior to impacting on the back of a piston.
Whichever method is used to pressurise the fluid, it is important that the peak pressure in the fluid is achieved quickly enough to enable the initial ‘pulse’ of fluid to have a sufficiently high pressure to penetrate the skin. The remainder of the fluid may be delivered at a similar pressure, or a substantially lower pressure, depending on the configuration of the device. Some needle free devices are designed to be filled by the user, whilst others are prefilled, either by the drug manufacturer or by a third party. In either case, it is important that the contents of the drug capsule are predominantly free from bubbles, especially in the nozzle area.